Relativistic Navigation Needed for Solar Sails

August 19, 2009

Relativistic Navigation Needed for Solar Sails

A decent-size solar sail could accelerate out of the solar system in no time, and this raises new challenges for navigators.

If we’re ever going to travel a significant distance
from Earth, we’re going to have to break our dependence on chemical propulsion
systems. It’s just not possible to carry enough chemical propellant to get up
to a decent trot.

An alternative is a solar sail, which uses the force of solar
radiation pressure to accelerate. By one calculation, a solar sail with a
radius of about a kilometer and a mass of 300 kg (including 150 kg of payload)
would have a peak acceleration of about 0.6 g if released on a parabolic
trajectory about 0.1 astronomical unit (AU) from the sun (where the radiation
pressure is higher).

That kind of acceleration would take it
beyond the Kuiper belt to the heliopause, the boundary between the solar system
and interstellar space (and a distance of 200 AU), in only 2.5 years.

In 30 years, a solar sail could travel
2,500 AU, far enough to explore the Oort Cloud.

But such a journey may not be smooth sailing,
particularly when it comes to navigation, say Roman Kezerashvili and Justin
Vazquez-Poritz, physicists at the City University of New York. They claim that
ordinary Newtonian physics just doesn’t cut it for this kind of journey.

The problem is that the sail would have
to be launched so close to the sun that the effects of general relativity, such
as the precession of the perihelion of orbiting objects, have to be taken into
account. And even though those effects are relatively minor to start with, they
have a significant effect over long distances.

The calculations carried out by
Kezerashvili and Vazquez-Poritz show that the effects of general relativity
could push a solar sail off course by as much as a million kilometers by the
time it reaches the Oort Cloud and that even tiny relativistic forces such as
frame dragging could cause a deflection of 1,000 kilometers.

What
these guys are saying is that the interstellar navigators will have to be proficient
in a new discipline of relativistic navigation.

That won’t be an issue for a while,
though. The most optimistic estimate for the launch of such a mission is around
2040.